1. Field of the Invention
The present invention relates to a semiconductor dynamic sensor such as an acceleration sensor for use in an automotive vehicle.
2. Description of Related Art
An example of an acceleration sensor formed on a silicon-on-insulator substrate (an SOI substrate) is disclosed in JP-A-9-113534. An essential portion of the acceleration sensor is shown in FIG. 22 attached hereto. A beam structure having a movable electrode 102 supported by four beams 101a, 101b, 101c and 101d is formed on an SOI substrate that includes a base substrate 100, an insulation layer and an upper semiconductor layer formed on the insulation layer. The four beams 10la–101d are supported on the insulation layer at each end thereof, and an air gap is formed between the movable electrode 102 and the base substrate 100.
When an accelerating force is imposed on the movable electrode 102, the movable electrode 102 moves in a direction (Z-direction shown in FIG. 22) perpendicular to the surface of the SOI substrate. A capacitance formed between the movable electrode 102 and the base substrate 100 varies according to an amount of acceleration imposed on the movable electrode 102. The amount of acceleration is detected based on an output representing the capacitance changes. The output is taken out from a terminal pad 100′ formed on the base substrate 100. However, the output of the acceleration sensor of this type is easily disturbed by noises. In addition, the terminal pad 100′ formed on the base substrate 100 has to be electrically connected to an outside circuit by wire-bonding.
Another example of an acceleration sensor is disclosed in JP-A-5-218300, the essence of which is shown in FIG. 23 attached hereto. The acceleration sensor is formed on a silicon substrate 110. On the silicon substrate 110, various layers and air gaps are formed in the following order: a first static conductor layer 111 (a poly-silicon layer), an air gap 112, a dynamic conductor layer 113 (a poly-silicon layer), another air gap 114, and a second static conductor layer 115 (a poly-silicon layer) . The first and the second static conductor layers 111, 115 are stationary, while the dynamic conductor layer 113 is movable in the direction perpendicular to the plane surface of the silicon substrate 110.
When an acceleration force is posed on the dynamic conductor layer 113, capacitances formed on both sides of the dynamic conductor layer 113 change according to the acceleration force. An amount of acceleration is detected based on an output signal representing capacitance difference between two capacitors. By using the capacitance difference between two capacitors, disturbing noises are canceled out in tow capacitors. However, it is not easy to manufacture this type of sensor because many layers have to be formed on the silicon substrate 110.